The sub wasn't steel, it was a carbon fibre weave. The tiniest crack would have caused catastrophic shattering of the hull.

For every ten meters of descent, pressure increases by 1 atmosphere. At 4000 meters, that's 400 atm, or ~40 Mega Pascals, which is significantly higher than the pressure caused by the bomb that leveled a building in Oklahoma.

The hull shattered catastrophically, and the pressure wave liquified the occupants instantly. It's horrifying, but at least it was painless.